When a crack or failed construction joint in a concrete structure needs to be sealed, the obvious choice is crack injection resin. There are many different types of crack injection resin available including epoxies and polyurethanes (PU).

The various products available all have their own particular properties and advantages, product selection will depend on many factors including the size of the crack and the amount of water flowing through it. Although there are many different resins available they will tend to be split into two main categories, resins for water bearing cracks and resins for dry cracks.

The resins available for water bearing or actively leaking cracks will be fast acting foams with free expansion of approx. 20-30 times. These resins are excellent for stopping leaks; they react with the water to foam within the crack, thus blocking the leakage path.

However it is very important to stress that these fast acting foams should not be considered as permanent solutions. During the foaming process an exothermic reaction is generated helping the individual cells within the foam to reach their maximum size.

During cooling and over the subsequent weeks a slight shrinkage of the foam will occur as it reaches its fully cured state. This shrinkage can lead to leaks between the foam and the surrounding concrete. Leaks could also occur where there is movement within the crack. Many water stopping foams are rigid and will not accommodate movement, some are flexible but will only resist a small amount of movement and not a regular expansion / contraction cycling.

Unfortunately the above properties of water stopping foams have led to a large number of failures. We often hear about projects where crack injection works have been “successfully” carried out only to have failed again within a few months, this is a shame because it gives the whole industry a bad reputation.

Responsible manufacturers normally recommend a two stage injection process for actively leaking cracks and joints. The first stage is injection with a fast acting water stopping foam as we have already described, however this is followed up with a secondary injecting with a flexible solid body resin.

The timing of the follow up injection will vary depending on the reaction time of the PU foam used in step 1, but the secondary injection should be carried out while the foam is still soft so typically this could be between 10 mins. and half an hour. This second injection process pushes back the foam within the crack replacing it with a material that will not shrink and will resist further movement.

Therefore a successful and permanent repair can be completed using a combination of products. Another option is to use a hybrid 2 in 1 style injection resin Such resins will foam on contact with water so can be considered as water blocking foams, but will also cure through to form an elastic solid body resin when no water contact is made. For a successful, permanent seal to be made a two stage injection process is still required, but in this instance the two stages can be carried out with the same product. This is of a particular advantage on site, where two different resins are no longer needed, the pump doesn’t need cleaning out between the two injection stages and the material mix. Is normally 1 part A to 1 part B (by volume) so it is very easy to get the mixing proportions correct. Another advantage of 2 in 1 type resins is that they work irrespective of water content within the crack so if the original site assessment is incorrect or the water loading of the crack has changed the same product will still work.

Previously if you selected a foam for a dry crack or a solid resin for a wet crack the repair would fail due to the foam not curing or the solid resin failing to bond to the sides of the crack. In summing up, we can conclude that it is always very important to understand the moisture loading and movement potential of a crack so that suitable repair materials can be selected.

When a crack is wet or actively leaking then a 2 stage injection process is necessary, either with a foam followed by a solid body resin or by a foam / solid body resin hybrid type product. A copy of this article and further information on crack injection resins can be found on the Koster website.